Ethylene glycols (monoethylene glycol, diethylene glycol and triethylene glycol) are prepared commercially by several methods. One of these methods involves a two-stage reaction system, the first stage of which requires the direct oxidation of ethylene with air or elemental oxygen over a suitable catalyst, typically a silver-containing catalyst, at elevated temperature (100.degree. to 500.degree.C. is typical) and at superatmospheric pressure (2 to 25 atmospheres).
Ethylene oxide produced in these reactors, which may be fixed or fluid bed reactors as typified by U.S. Pat. Nos. 2,125,333 and 2,430,443, is removed from the reactors in a gas stream and is passed into an absorbing vessel where it is contacted with water to absorb the ethylene oxide content thereof. The gases (which still contain appreciable quantities of ethylene) are then recycled to the reactor while the ethylene oxide containing water in the absorbing vessel is passed to a stripping column. In the stripping column steam or hot water is introduced and contacted usually countercurrent to the ethylene oxide fed thereto to remove ethylene oxide product overhead. The water discharged from the stripping vessel is recirculated to the absorbing vessel for use in absorbing ethylene oxide therein.
In other systems a steam heated reboiler is employed to heat water in the bottom of the stripping column and boil it. This generates steam internally in the stripping column. While this eliminates a water build-up problem glycols still accumulate in the column and must be purged to the evaporation and/or recovery systems of the glycol producing unit.
As will be readily understood by the skilled artisan, the introduction of water into the stripping column and the closed recycle system between the stripping vessel and the absorbing vessel causes a buildup of water in the system requiring a purge to remove excess water. This purge stream contains appreciable quantities of ethylene glycol in it and is usually of such value that it cannot be discarded. Further, since it contains ethylene glycol, it cannot be easily disposed of due to the fact that ethylene gylcol has a deleterious effect of the total oxygen demand of bodies of water in which this material might be discharged. Similarly, where heated stripping vessels are used glycol buildup in the columns required a bleed of water-glycol from the system.
In a typical glycol plant in which ethylene oxide is hydrolyzed in a reaction zone with water at elevated temperature and pressure as the second stage of a reaction system and in which system glycols are subsequently evaporated and distilled to produce the pure glycols those waste streams are fed to the glycol reaction system to avoid the overall loss of glycols that accompanies disposal of glycol containing streams. These streams may be fed to any of several places in this second stage reaction system which is typically provided with a glycol reactor in which the oxide feed is converted to glycols, an evaporation system in which the glycols are concentrated and a still system in which all water is removed and in which monethylene glycol, diethylene glycol and triethylene glycol are separated from each other.
It has been found that the glycols produced when these water streams containing ethylene glycol are utilized in the glycols reaction system usually have an ultraviolet light absorption characteristic such that they are not suitable for use in the manufacture of polyester fibers. This limits the use of the product. Polyester fibers may be produced by reaction of ethylene glycol and terephthalic acid for example and limitation on the use of the glycols produced for this purpose presents a serious problem to the producer of glycols since a significant and desirable product market is thus lost.